Russian President Vladimir Putin on Saturday said an "armed mutiny" by the Wagner Group mercenary force was treason, and that anyone who had taken up arms against the Russian military would be punished.
Mr Putin said decisive actions will be taken to stabilise the situation in the Russian city of Rostov-on-Don after Wagner chief Yevgeny Prigozhin claimed that his forces had taken control of all key military sites there, Reuters reported.
The authorities in Moscow and its surrounding region declared a counterterrorism state of emergency.
“In order to prevent possible terrorist attacks in the city and Moscow region, a regime of counterterrorism operations has been established,” Russia’s National Antiterrorism Committee said.
Mr Prigozhin had claimed in a new video message posted on Telegram that all military sites Rostov-on-Don are under the Wagner mercenary group’s control.
He also said that this does not impede on Russia’s conduct of what it calls a “special military operation” in Ukraine.
Moscow authorities ramped up security measures on Friday, the state-run TASS news agency reported, due to concerns over Wagner Group leader Yevgeny Prigozhin's vows to attack Russia's military for an alleged offensive against his force.
Heightened protection is now in place for critical and government infrastructure, as well as public transportation, in Russia's capital hours after Mr Prigozhin posted a video claiming the Ministry of Defence launched fatal missile attacks on his paramilitary group.
The Wagner chief, who is central to many significant conflicts in Eastern Ukraine and elsewhere, declared an operation to reclaim “justice” and demanded an overhaul in the defence ministry's leadership.
The move, described as the most audacious challenge to President Vladimir Putin since the war in Ukraine began, has led to Russian authorities opening a criminal investigation into Mr Prigozhin for “armed mutiny”.
The country's Federal Security Service (FSB) accused Mr Prigozhin of inciting a “civil conflict” and called upon Wagner fighters to arrest him via a special broadcast on Russian state TV.
Video evidence of armoured vehicles being deployed across the capital, including near the defence ministry, has been posted by local media.
Mr Prigozhin claimed on Saturday that a Wagner convoy was making its way into Rostov, a southern city in Russia near the border of Ukraine, but there has been no evidence backing his statements.
The White House National Security Council's spokesperson said that President Joe Biden has been briefed on the situation and that the US is "monitoring the situation and will be consulting with allies and partners on these developments".
Ukraine's Ministry of Defence tweeted: "We are watching."
In response to Mr Prigozhin's accusations, Gen Sergey Surovikin urged Wagner fighters to obey the will of the President and return to their bases.
The FSB also appealed to Wagner members to disregard Mr Prigozhin's orders, deeming them “criminal and traitorous”, and called on them to take steps to apprehend him.
Mr Prigozhin accused the Russian Ministry of Defence of orchestrating strikes on the group's encampments, inflicting a “huge” number of fatalities on their forces.
He also launched a vehement critique of Moscow's military operations and the rationale behind Russia's invasion of Ukraine.
The Wagner chief, who has had a fractious relationship with Russia's defence leaders, has accused the ministry and top general, Valery Gerasimov, of gross incompetence.
He dismissed Russia's justifications for the invasion of Ukraine, which Moscow refers to as a “special military operation”.
Mr Prigozhin released a video clip on Telegram through his press service, expressing his scorn for the official narrative.
“The Defence Ministry is trying to deceive society and the president and tell us a story about how there was crazy aggression from Ukraine and that they were planning to attack us with the whole of Nato,” Mr Prigozhin said.
He continued his indictment of Russia's military motivations, alleging that the war was not about the demilitarisation or denazification of Ukraine, as publicly stated by Russian authorities.
Instead, he claimed it was fuelled by the vanity of Defence Minister Sergey Shoigu and the unquenchable desire for wealth by the ruling elite.
Furthermore, he accused Russia's military leadership of hampering the war effort through rampant corruption and accused leaders of wanton disregard for the lives of young soldiers.
This public condemnation came against the backdrop of an order by Mr Putin and the ministry for mercenary groups such as Wagner to be brought under ministry control, a directive that Mr Prigozhin has vehemently resisted.
Meanwhile, Ukraine continues its counter-offensive against Russian forces.
Despite having reclaimed eight villages since the start of its campaign, President Volodymyr Zelenskyy stated that progress was “slower than desired”.
Ukrainian officials have emphasised that the full-scale counter-offensive has yet to be launched.
Presidential adviser Mykhailo Podolyak tweeted: “The counter-offensive is not a new season of a Netflix show.
“There is no need to expect action and buy popcorn.”
This sentiment was echoed by deputy defence minister Hanna Maliar, who affirmed that the “main events” of the counter-offensive were “ahead of us”.
Patrick deHahn in New York contributed reporting